Thermogravimetric Analysis (TGA): Principle and Uses

Thermogravimetric analysis (TGA) weighs a sample continuously while it is heated at a controlled rate. A highly sensitive microbalance records the sample's mass as temperature rises, so any change that releases matter — drying, decomposition or oxidation — appears as a step in the mass reading. The result is a thermogram: a plot of mass (or mass percent) against temperature.

TGA matters because it shows exactly when and how a material breaks down — drug companies check medicine purity, polymer makers study how plastics degrade with heat, and geologists identify minerals by their decomposition steps.

Key Concepts

• Microbalance: the heart of the instrument, detecting mass changes of about a microgram while the sample sits in a furnace.
• Heating programme: the temperature schedule, usually a steady ramp such as 10°C per minute, in a chosen gas (nitrogen for decomposition, air for oxidation studies).
• Thermogram: the mass-versus-temperature curve. Flat plateaus mean the sample is stable; downward steps mean gases are escaping.
• Onset temperature: where decomposition begins, found by extending the flat baseline and the steep part of the step until they meet.
• Decomposition steps: many compounds lose mass in stages — first loosely bound water, then more tightly held groups — and each step can confirm a chemical formula.
• DTG (derivative thermogravimetry): the rate of mass loss versus temperature. Its peaks mark the fastest decomposition and separate overlapping steps that blur together on the plain TGA curve.

Formulas

• Mass-loss percent: % loss = (mass lost / initial mass) × 100, compared with the theoretical percent from the chemical formula to identify what left.
• Calcium oxalate monohydrate (CaC2O4·H2O, 146.12 g/mol): losing one H2O (18.02 g/mol) gives 18.02/146.12 × 100 = 12.33%; losing CO (28.01 g/mol) from CaC2O4 gives 28.01/146.12 × 100 = 19.17%; losing CO2 (44.01 g/mol) from CaCO3 gives 44.01/146.12 × 100 = 30.12%. Final residue CaO = 38.38%.
• Assumptions and limits: each step must belong to one reaction; overlapping reactions blur together, and hot-gas buoyancy shifts apparent mass slightly (corrected with blank runs).

Worked Example

A 100.0 mg sample of calcium oxalate monohydrate is heated at 10°C/min in nitrogen. Step 1 (100–200°C): 100.0 → 87.7 mg, a 12.3% loss matching the theoretical 12.33% for one H2O (CaC2O4·H2O → CaC2O4 + H2O). Step 2 (400–500°C): 87.7 → 68.5 mg, a 19.2% loss matching the theoretical 19.17% for CO (CaC2O4 → CaCO3 + CO). Step 3 (650–850°C): 68.5 → 38.4 mg, a 30.1% loss matching the theoretical 30.12% for CO2 (CaCO3 → CaO + CO2). The residue is 38.4% CaO, and the DTG curve shows three clean peaks, one per step.

Common Misconceptions

• "Every step means one compound leaving." Two reactions can overlap and look like one step; the DTG curve splits them.
• "Onset temperature is a fixed property." It shifts with heating rate and sample size — faster heating pushes it higher, so conditions must be reported and kept constant.
• "TGA identifies the gases." TGA only measures mass loss. Identifying the gas needs another technique or a match to a known formula.

Real-World Applications

• Polymers: thermal stability, filler content (the ash left at 800°C) and degradation temperatures.
• Pharmaceuticals: water content in drugs and excipient compatibility checks.
• Cement and minerals: quantifying carbonates, clays and hydrated phases by their characteristic steps.
• Food: moisture content in grains and thermal behaviour of starches and oils.

TGA weighs a sample as it heats, turning decomposition into a staircase of mass loss sharpened by the DTG derivative. The calcium oxalate curve — 12.33% water, 19.17% CO, 30.12% CO2 — is the textbook standard for calibrating instruments.